Electronic device and electronic system

ABSTRACT

One object of an embodiment is to provide a novel electronic device which is configured so that a user can read data regardless of a location, input data by directly touching a keyboard displayed on a screen or indirectly touching the keyboard with a stylus pen or the like, and use the input data. The electronic device includes a first transistor electrically connected to a reflective electrode and a photo sensor over a flexible substrate. A touch-input button is displayed as a still image on a first screen region of a display portion, and a video signal is output so that a moving image is displayed on a second screen region of the display portion. A video signal processing portion supplying different signals between the case where a still image is displayed on the display portion and the case where a moving image is displayed on the display portion is provided. After writing of a still image is performed, a display element control circuit is put in a non-operation state, whereby power consumption can be reduced.

TECHNICAL FIELD

The present invention relates to an electronic device having a circuitincluding a transistor, and also relates to an electronic system. Forexample, the present invention relates to an electronic device on whichan electro-optical device typified by a liquid crystal display panel ismounted as a component.

BACKGROUND ART

In recent years, display devices such as electronic book readers havebeen actively developed. In particular, since a technique in which animage is displayed with the use of a display element having memoryproperties greatly contributes to reduction of power consumption, thetechnique has been actively developed (Patent Document 1).

In addition, a display device provided with a touch sensor has attractedattention. The display device provided with a touch sensor is called atouch panel, a touch screen, or the like (hereinafter also referred tosimply as a touch panel). Further, a display device on which an opticaltouch sensor is mounted is disclosed in Patent Document 2.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-267982-   [Patent Document 2] Japanese Published Patent Application No.    2001-292276

DISCLOSURE OF INVENTION

An object of an embodiment is to provide an electronic device which islightweight and flexible and whose impact resistance is improved byusing a flexible film as a base instead of a hard substrate such as aglass substrate.

Further, an object of an embodiment is to provide a driver circuit whichis advantageous for energy saving of an electronic device whose power islimited, such as a portable information terminal.

One object of an embodiment is to provide a novel electronic devicewhich is configured so that a user can read data regardless of alocation, input data by directly touching a keyboard displayed on ascreen or indirectly touching the keyboard with a stylus pen or thelike, and use the input data. An object of an embodiment is to provide apixel structure in which a light-receiving area of photo sensors and apixel electrode area per unit area are increased in order to obtain anelectronic device configured so that a user can read data and input databy touching a keyboard displayed on a screen.

In addition, an object of an embodiment is to provide a novel electronicdevice in which a still-image mode including display of a keyboard and amoving-image mode are realized on one screen.

In addition, one object of an embodiment is to reduce power consumptionin such a manner that, in the case of the still-image mode, a stillimage is displayed on part of a display portion, then supply of power todisplay elements in a region where the still image is displayed isstopped, and a state in which the still image can be seen is kept for along time after the stop of supply.

In an electronic device including a display portion in which an image isdisplayed using external light, the display portion has a touch-inputfunction with the use of photo sensors, keyboard buttons are displayedon at least part of the display portion, and a user inputs data bytouching a desired key, so that display corresponds to the desired keyis performed on the display portion.

The photo sensors detect external light entering the display portion anda shadow which is made on part of the display portion (hereinafter, alsoreferred to as a partial shadow of external light) when a user points adesired position on the display device. An input processing portionprocesses the position of a photo sensor which detects the partialshadow of external light on the display portion, as the coordinateposition of touch input. A video signal processing portion outputs datacorresponding to the coordinate of touch input, i.e., data of akeyboard, as image data to the display portion.

A first display region on which the keyboard is displayed displays astill image in a period in which a user inputs data with the keyboarddisplayed on the display portion. When the user inputs data, a seconddisplay region displays a moving image in a period in which letters ornumerals corresponding to keys touched are written one after another orin a period in which conversion of letters is performed.

An embodiment of the present invention disclosed in this specificationis an electronic device including a video signal processing portionwhich switches a first screen region of a display portion to a screenregion on which touch input is performed or a screen region on whichoutput is performed to display. Alternatively, the electronic deviceincludes a video signal processing portion which supplies differentsignals to display elements of the display portion between the casewhere a still image is displayed on the display portion and the casewhere a moving image is displayed on the display portion. After writingof a still image is performed, a display element control circuit is putin a non-operation state, whereby power consumption can be reduced. Inparticular, a decoder circuit is preferably used as a scan line drivercircuit.

A switching transistor included in a conventional active matrix displaydevice has a problem in that off current is large and a signal writtento a pixel leaks to disappear in the transistor even when the transistoris in an off state. According to an embodiment of the present invention,with the use of a transistor including an oxide semiconductor layer as aswitching transistor, extremely small off current, specifically, offcurrent density per channel width of 1 μm at room temperature can beless than or equal to 10 aA (1×10⁻¹⁷ A/μm), further, less than or equalto 1 aA (1×10⁻¹⁸ A/μm), or still further, less than or equal to 10 zA(1×10⁻²⁰ A/μm). In addition, in the pixel, holding time of an electricsignal such as an image signal can be longer, and intervals of writingtime can be set long. Therefore, with the use of the transistorincluding an oxide semiconductor, a period in which the display elementcontrol circuit in a non-operation state after writing of a still imageis prolonged, whereby power consumption can be further reduced.

An embodiment of the present invention relating to a device to realizean electronic device is an electronic device comprising a displayportion having a touch-input function; and a first transistorelectrically connected to a reflective electrode, and a photo sensorover a flexible substrate. In the electronic device, the photo sensorcomprises a photodiode, a second transistor including a gate signal lineelectrically connected to the photodiode, and a third transistor. In theelectronic device, one of a source and a drain of the second transistoris electrically connected to a photo sensor reference signal line, theother of the source and the drain of the second transistor iselectrically connected to one of a source and a drain of the thirdtransistor, and the other of the source and the drain of the thirdtransistor is electrically connected to a photo sensor output signalline.

With the above structure, at least one of the above problems can beresolved.

In the above structure, an oxide semiconductor layer of the secondtransistor overlaps with a reading signal line with a gate insulatinglayer provided therebetween, and the reading signal line overlaps withthe reflective electrode that is a pixel electrode. With a pixelstructure in which the reading signal line and the third transistor areprovided below the reflective electrode, a light-receiving area of photosensors and a pixel electrode area (hereinafter, referred to as areflective electrode area) per unit area can be effectively used.

In addition, an embodiment of the invention is an electronic devicecomprising a display portion having a touch-input function; and a firsttransistor electrically connected to a first reflective electrode, asecond transistor electrically connected to a second reflectiveelectrode, and a photo sensor, over a flexible substrate. In theelectronic device, the photo sensor comprises a photodiode, a thirdtransistor including a gate signal line electrically connected to thephotodiode, and a fourth transistor. In the electronic device, one of asource and a drain of the third transistor is electrically connected toa photo sensor reference signal line, the other of the source and thedrain of the third transistor is electrically connected to one of asource and a drain of the fourth transistor, and the other of the sourceand the drain of the fourth transistor is electrically connected to aphoto sensor output signal line. In the electronic device, an oxidesemiconductor layer of the fourth transistor overlaps with the firstreflective electrode, and the photo sensor reference signal lineoverlaps with the second reflective electrode.

The above structure is designed so that one light-receiving region of aphoto sensor is provided between two reflective electrodes when thepixel structure is seen from above, whereby a light-receiving area ofphoto sensors and a reflective electrode area per unit area can beeffectively used.

In the above structure, an oxide semiconductor layer of the thirdtransistor overlaps with a reading signal line with a gate insulatinglayer provided therebetween, and the reading line overlaps with thefirst reflective electrode. With a pixel structure in which the readingsignal line and the fourth transistor are provided below the firstreflective electrode, a light-receiving area of photo sensors and areflective electrode area per unit area can be effectively used.

In either of the above structures, one of the source and the drain ofthe fourth transistor overlaps with the first reflective electrode andthe other of the source and the drain of the fourth transistor overlapswith the second reflective electrode. With such a pixel structure, alight-receiving area of photo sensors and a reflective area per unitarea can be effectively used.

In addition, in the above structure, a color filter is provided tooverlap with the first reflective electrode or the second reflectiveelectrode, whereby full-color display can also be performed.

Further, a reflective liquid crystal device is advantageous for energysaving because displayed contents can be recognized with external lightsuch as sunlight or illumination light even a backlight is not provided.

A portable information terminal in which a moving image and a stillimage are displayed on one screen can be realized. Driving and supply ofa signal are performed in a different manner between in the case where amoving image is displayed on a screen region and the case where a stillimage is displayed on a screen region, and power consumption fordisplaying a moving image is reduced as compared to that for displayinga still image. In addition, since a reflective liquid crystal displaydevice is employed, halftone display in grayscale with a wider range ofgradation than in the case of an electrophoretic display device can beperformed.

In addition, with the use of a potable information terminal whose weightis reduced by using a flexible substrate, a user can read dataregardless of a location, and input data by touching a keyboarddisplayed on a screen, so that a result of the input can be displayed onthe screen on which the keyboard is displayed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are external views illustrating an embodiment of thepresent invention;

FIG. 2 is a block diagram illustrating an embodiment of the presentinvention;

FIG. 3 is an equivalent circuit diagram of a pixel, illustrating anembodiment of the present invention;

FIG. 4 is a schematic view of a driver circuit for photo sensors,illustrating an embodiment of the present invention;

FIG. 5 is a timing chart showing an embodiment of the present invention;

FIG. 6 is an example of a plan view of a pixel, illustrating anembodiment of the present invention;

FIG. 7 is an example of a plan view showing a positional relationbetween a reflective electrode and a black matrix, illustrating anembodiment of the present invention;

FIGS. 8A and 8B are an example of cross-sectional views illustrating anembodiment of the present invention;

FIG. 9 is a schematic view of a liquid crystal display module,illustrating an embodiment of the present invention;

FIGS. 10A and 10B are an external view and a block diagram of a displaydevice which is an embodiment of the present invention;

FIGS. 11A and 11B are a photograph of a cross section of part of aphotodiode and a schematic view thereof;

FIG. 12 is a photograph showing a state of a display panel displaying animage;

FIGS. 13A to 13C are an example of a cross-sectional views illustratingan embodiment of the present invention;

FIGS. 14A and 14B are schematic views of an electronic book reader,illustrating an embodiment of the present invention;

FIG. 15 is a block diagram of a driver circuit, illustrating anembodiment of the present invention;

FIG. 16 shows a relation among data lines with respect to time;

FIG. 17 is a diagram of a circuit forming the inside of a block; and

FIG. 18 shows time and respective potentials of nodes.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, an example of an electronic device including adisplay portion in which an image is displayed using external light isdescribed with reference to FIGS. 1A and 1B.

A display portion 1032 of an electronic device 1030 has a touch-inputfunction with the use of photo sensors, in which a plurality of keyboardbuttons 1031 is displayed on a region 1033 in the display portion asillustrated in FIG. 1A. The display portion 1032 indicates the entiredisplay region and includes the region 1033 in the display portion. Auser inputs data by touching desired keyboard buttons, whereby a resultof the input is displayed on the display portion 1032.

Since the region 1033 in the display portion displays a still image,power consumption can be reduced by putting a display element controlcircuit in a non-operation state in a period other than writing time.

An example of the usage of the electronic device 1030 is described. Forexample, letters are input by successively touching the keyboard buttonsdisplayed on the region 1033 in the display portion with user's fingersor without contact, and text which is displayed as a result of the inputis displayed on a region other than the region 1033 in the displayportion. After a set period of time in which an output signal of thephoto sensor is not detected since the user removes his/her fingers fromthe keyboard of the screen, the keyboard displayed on the region 1033 inthe display portion is removed automatically and the input text isdisplayed also on the region 1033 in the display portion, so that theuser can see the input text displayed on all the region of the screen.In the case where input is performed again, the keyboard buttons can bedisplayed on the region 1033 in the display portion again by touch ofthe display portion 1032 with the user's finger or detection of anoutput signal of a photo sensor without contact, and input of letterscan be performed.

Alternatively, the keyboard can be removed not automatically but bypushing a switch 1034 by the user so that a still image can be displayedon the entire display portion 1032 as illustrated in FIG. 1B. Inaddition, even when the power is turned off by pushing a power supplyswitch 1035, the still image can be held for a long time. Further, thekeyboard can be displayed by pushing a keyboard display switch 1036 tobe in a state where touch-input can be performed.

In addition, the switch 1034, the power supply switch 1035, and thekeyboard display switch 1036 may each be displayed on the displayportion 1032 as a switch button. Each operation may be performed bytouching the displayed switch button.

Further, without limitation to the structure in which the region 1033 inthe display portion displays a still image, the region 1033 in thedisplay portion may display a moving image temporarily or partly. Forexample, a position where the keyboard buttons are displayed may bechanged temporarily in accordance with user's taste, or when input isperformed without contact, only display of a keyboard buttoncorresponding to a letter which is input may be partly changed in orderto confirm that whether the input is performed.

The electronic device 1030 includes at least a battery, and preferablyincludes a memory for storing data (e.g., a flash memory circuit, anSRAM circuit, or a DRAM circuit), a central processing unit (CPU), or alogic circuit. With a CPU or a memory, various kinds of software can beinstalled and thus, the electronic device 1030 can have part or all ofthe functions of a personal computer.

In addition, by providing a gradient detection portion such as agyroscope or a triaxial acceleration sensor in the electronic device1030, a function used in the electronic device 1030, in particular, afunction relating to display and input on the display portion can beswitched by an arithmetic circuit in accordance with a signal from thegradient detection portion. Therefore, unlike an electronic device withan input key which has predetermined kind, size, or arrangement, such asa built-in operation key, the electronic device 1030 can improveconvenience for users.

Next, an example of a display panel included in the display portion 1032is described with reference to FIG. 2. A display panel 100 includes apixel circuit 101, a display element control circuit, and a photo sensorcontrol circuit. The pixel circuit 101 includes a plurality of pixels103 and 104 and a plurality of photo sensors 106 which are arranged in amatrix of rows and columns. Each of the pixels 104 and 103 includes onedisplay element. Although in this embodiment, one photo sensor 106 isprovided between the pixel 103 and the pixel 104 and the number of thephoto sensors is half of the number of the pixels, an embodiment is notlimited thereto. One photo sensor may be provided per one pixel so thatthe number of the photo sensors is the same as the number of the pixels.Alternatively, the number of the photo sensors may be one third of thenumber of the pixels.

A display element 105 includes a liquid crystal element including atransistor, a storage capacitor, and a liquid crystal layer, or thelike. The transistor has a function of controlling injection ordischarge of charge to/from the storage capacitor. The storage capacitorhas a function of retaining charge which corresponds to voltage appliedto the liquid crystal layer. Taking advantage of the change in thedirection of a polarization due to a voltage application to the liquidcrystal layer, contrast (gray scale) of light passing through the liquidcrystal layer is made, so that image display is realized. External light(sunlight or illumination light) which enters from a surface side of aliquid crystal display device is used as the light passing through theliquid crystal layer. There is no particular limitation on the liquidcrystal layer, and a known liquid crystal material (typically, a nematicliquid crystal material or a cholesteric liquid crystal material) may beused. For example, polymer dispersed liquid crystal (PDLC) or polymernetwork liquid crystal (PNLC) may be used for the liquid crystal layerso that white display (light display) is performed using scattered lightby liquid crystal. When PDLC or PNLC is used for the liquid crystallayer, a polarizing plate is not needed and display close to paper ispossible, which is eye-friendly and causes less feeling tired.

Further, the display element control circuit is a circuit configured tocontrol the display elements 105 and includes a display element drivercircuit 107 which inputs a signal to the display elements 105 throughsignal lines (also referred to as source signal lines) such as videodata signal lines, and a display element driver circuit 108 which inputsa signal to the display elements 105 through scan lines (also referredto as gate signal lines).

For example, the display element driver circuit 108 on the scan lineside has a function of selecting the display elements included in thepixels placed in a particular row. The display element driver circuit107 on the driving the signal line side has a function of applying apredetermined potential to the display elements included in the pixelsplaced in a selected row. Note that in the display element to which thedisplay element driver circuit 108 on the scan line side applies highpotential, the transistor is in a conduction state, so that the displayelement is supplied with charge from the display element driver circuit107 on the signal line side

The photo sensor 106 includes a transistor and a light-receiving elementwhich has a function of generating an electrical signal when receivinglight, such as a photodiode.

The photo sensor control circuit is a circuit configured to control thephoto sensors 106 and includes a photo sensor reading circuit 109 on thesignal line side for a photo sensor output signal line, a photo sensorreference signal line, or the like, and a photo sensor driver circuit110 on the scan line side. The photo sensor driver circuit 110 on thescan line side has a function of performing reset operation andselecting operation on the photo sensors 106 included in the pixelsplaced in a particular row, which is described below. Further, the photosensor reading circuit 109 on the signal line side has a function oftaking out an output signal of the photo sensors 106 included in thepixels in the selected row.

A circuit diagram of the pixel 103, the photo sensor 106, and the pixel104 is described in this embodiment with reference to FIG. 3. The pixel103 includes the display element 105 including a transistor 201, astorage capacitor 202, and a liquid crystal element 203. The photosensor 106 includes a photodiode 204, a transistor 205, and a transistor206. The pixel 104 includes a display element 125 including a transistor221, a storage capacitor 222, and a liquid crystal element 223.

A gate of the transistor 201 is electrically connected to a gate signalline 207, one of a source and a drain of the transistor 201 iselectrically connected to a video data signal line 210, and the other ofthe source and the drain of the transistor 201 is electrically connectedto one electrode of the storage capacitor 202 and one of electrodes ofthe liquid crystal element 203. The other electrode of the storagecapacitor 202 is electrically connected to a capacitor wiring 214 andheld at a fixed potential. The other electrode of the liquid crystalelement 203 is held at a fixed potential. The liquid crystal element 203is an element including a pair of electrodes and a liquid crystal layerprovided between the pair of electrodes.

When “H” (high-level potential) is applied to the gate signal line 207,the transistor 201 applies the potential of the video data signal line210 to the storage capacitor 202 and the liquid crystal element 203. Thestorage capacitor 202 holds the applied potential. The liquid crystalelement 203 changes light transmittance in accordance with the appliedpotential.

One of electrodes of the photodiode 204 is electrically connected to aphotodiode reset signal line 208, and the other electrode of thephotodiode 204 is electrically connected to a gate of the transistor205. One of a source and a drain of the transistor 205 is electricallyconnected to a photo sensor reference signal line 212, and the other ofthe source and the drain of the transistor 205 is electrically connectedto one of a source and a drain of the transistor 206. A gate of thetransistor 206 is electrically connected to a reading signal line 209,and the other of the source and the drain of the transistor 206 iselectrically connected to a photo sensor output signal line 211.

A gate of the transistor 221 is electrically connected to a gate signalline 227, one of a source and a drain of the transistor 221 iselectrically connected to the video data signal line 210, and the otherof the source and the drain of the transistor 221 is electricallyconnected to one of electrodes of the storage capacitor 222 and one ofelectrodes of the liquid crystal element 223. The other electrode of thestorage capacitor 222 is electrically connected to a capacitor wiring224 and held at a fixed potential. The other electrode of the liquidcrystal element 223 is held at a fixed potential. The liquid crystalelement 223 includes a pair of electrodes and a liquid crystal layerprovided between the pair of electrodes.

Next, an example of a structure of the photo sensor reading circuit 109is described with reference to FIG. 3 and FIG. 4. For example, thedisplay portion includes pixels provided in 1024 rows and 768 columns.One display element is provided in one pixel in each row and each columnand one photo sensor is provided between two pixels in two rows and onecolumn. That is, the display elements are provided in 1024 rows and 768columns, and the photo sensors are provided in 512 rows and 768 columns.In addition, this embodiment describes an example in which output to theoutside of the display device is performed in the case where photosensor output signal lines in two columns are regarded as one pair. Thatis, one output is obtained from two photo sensors provided between fourpixels in two rows and two columns.

FIG. 3 illustrates a circuit configuration of pixels, in which twopixels and one photo sensor for two rows and one column are illustrated.One display element is provided in one pixel and one photo sensor isprovided between two pixels. FIG. 4 illustrates a circuit configurationof the photo sensor reading circuit 109, in which some photo sensors areillustrated for explanation.

As illustrated in FIG. 4, an example of a driving method is considered,in which a scan line driver circuit for photo sensors drives pixels forfour rows (that is, photo sensors for two rows) concurrently, and shiftsselected rows by one row including photo sensors corresponding to pixelsfor two rows. Here, photo sensors in each row are continually selectedin a period in which the scan line driver circuit shifts selected rowstwice. Such a driving method facilitates improving frame frequency atthe time of imaging by a photo sensor. In particular, it is advantageousin the case of a large-sized display device. Note that outputs of photosensors in two rows are superimposed on the photo sensor output signalline 211 at one time. All of the photo sensors can be driven byrepeating shift of selected rows 512 times.

As illustrated in FIG. 4, in the photo sensor reading circuit 109, oneselector is provided per pixels for 24 rows. The selector selects 1 pairfrom 12 pairs of photo sensor output signal lines 211 (1 paircorresponds to photo sensor output signal lines 211 for two columns) inthe display portion and obtains an output. In other words, the photosensor reading circuit 109 includes 32 selectors in total and obtains 32outputs at one time. Selection is performed on all of the 12 pairs ineach selector, whereby 384 outputs which correspond to one row of photosensors can be obtained in total. The selector selects 1 pair from the12 pairs every time selected rows are shifted by the scan line drivercircuit of photo sensors, whereby outputs from all of the photo sensorscan be obtained.

In this embodiment, as illustrated in FIG. 4, the following structure isconsidered: the photo sensor reading circuit 109 on the signal line sidetakes out outputs of photo sensors, which are analog signals, to theoutside of the display device, and the outputs are amplified with theuse of an amplifier provided outside the display device and converted todigital signals with the use of an AD converter. Needless to say, thefollowing structure may be employed: the AD converter is mounted on asubstrate over which the display device is provided, and the outputs ofphoto sensors are converted to digital signals and then the digitalsignals are taken out to the outside of the display device.

In addition, operation of individual photo sensors is realized byrepeating reset operation, accumulating operation, and selectingoperation. The “reset operation” refers to operation in which thepotential of the photodiode reset signal line 208 is set to “H”. Whenthe reset operation is performed, the photodiode 204 is brought intoconduction, and the potential of a gate signal line 213 to which thegate of the transistor 205 is connected is set to “H”.

The “accumulating operation” refers to operation in which the potentialof the photodiode reset signal line 208 is set to “L” (low-levelpotential) after the reset operation. Further, the “selecting operation”refers to operation in which the potential of the reading signal line209 is set to “H” after the accumulating operation.

When the accumulating operation is performed, the potential of the gatesignal line 213 to which the gate of the transistor 205 is connected isreduced as light with which the photodiode 204 is irradiated isstronger, and a channel resistance of the transistor 205 is increased.Therefore, when the selecting operation is performed, a current whichflows to the photo sensor output signal line 211 through the transistor206 is small. On the other hand, as the light with which the photodiode204 is irradiated is weaker at the time of the accumulating operation, acurrent which flows to the photo sensor output signal line 211 throughthe transistor 206 is increased at the time of the selecting operation.

In this embodiment, when the reset operation, the accumulatingoperation, and the selecting operation are performed on the photosensors, a partial shadow of external light can be detected. Inaddition, when image processing or the like is performed on the detectedshadow appropriately, a position where a finger, a stylus pen, or thelike touches the display device can be recognized. Operationcorresponding to the position where the display device is touched, forexample, as for input of letters, kinds of letters are regulated inadvance, so that desired letters can be input.

Note that in the display device in this embodiment, the partial shadowof external light is detected by the photo sensors. Therefore, even if afinger, a stylus pen, or the like does not touch the display devicephysically, when the finger, the stylus pen, or the like gets close tothe display device without contact and a shadow is formed, detection ofthe shadow is possible. Hereinafter, “a finger, a stylus pen, or thelike touches the display device” includes the case where the finger, thestylus pen, or the like is close to the display device without contact.

With the above structure, the display portion 1032 can have atouch-input function.

In the case where touch input is performed, the display device has astructure in which an image partly including a still image such as akeyboard is displayed and input is performed by touching a positionwhere a keyboard or a desired letter is displayed with a finger or astylus pen, whereby operability is improved. In the case where such adisplay device is realized, power consumption in the display device canbe considerably reduced in the following manner. That is, in a firstscreen region where the still image is displayed on the display portion,it is effective that supply of power to display elements in the firstscreen region is stopped after the still image is displayed and that astate in which the still image can be seen is kept for a long time afterthe stop of supply. In a second screen region that is the rest of thedisplay portion, a result from the touch input is displayed, forexample. The display element control circuit is in a non-operation statein a period other than the time of updating the displayed image in thesecond screen region, whereby power can be saved. A driving method whichenables the above control is described below.

For example, FIG. 5 shows a timing chart of a shift register of the scanline driver circuit in the display device including the display portionin which the display elements are arranged in 1024 rows and 768 columns.A period 61 in FIG. 5 corresponds to one cycle period (64.8 μsec) of aclock signal. A period 62 corresponds to a period (8.36 msec) which isneeded for finishing writing of the display elements from a 1st to 512throws corresponding to the second screen region. A period 63 correspondsto one frame period (16.7 msec).

Here, the shift register of the scan line driver circuit is afour-phase-clock-type shift register which is operated by a first clocksignal CK1 to a fourth clock signal CK4. In addition, the first clocksignal CK1 to the fourth clock signal CK4 are sequentially delayed by aquarter of one cycle period. When a start pulse signal GSP is set to“H”, a gate signal line G1 in a 1st row to a gate signal line G512 in a512th row are set to “H” in sequence with a delay of a quarter of onecycle period. In addition, each of the gate signal lines is set at “H”during half of one cycle period, and two gate signal lines in successiverows are set at “H” concurrently during quarter of one cycle period.

Here, the display elements in each row are continuously selected in aperiod in which the scan line driver circuit shifts selected rows twice.When data of the displayed image are input in the latter half of aperiod in which the display elements in the row are selected, thedisplayed image can be updated.

Here, in a period other than a period in which the displayed image bythe display elements from the 1st row to the 512th row which correspondto the second screen region is updated, the display element controlcircuit is in a non-operation state. That is, the displayed image bydisplay elements in a 513th row to a 1024th row corresponding to thefirst screen region is not updated and the display element controlcircuit is in a non-operation state.

The non-operation state of the display element control circuit can berealized by stopping the clock signal (keeping a clock signal at apotential “L”) as shown in FIG. 5. It is effective to stop of supply ofpower supply voltage at the same time as the stop of the clock signal.

In addition, supply of a clock signal and a start pulse signal may bestopped also in the driver circuit on the source in a period in whichthe display elements corresponding to the second screen region are notselected, that is, in a period in which the displayed image is notupdated. In such a manner, power can be further saved.

Further, power can be saved by using a decoder instead of the shiftregister of the scan line driver circuit.

Embodiment 2

A pixel structure corresponding to FIG. 2 and FIG. 3 described inEmbodiment 1 is described in this embodiment with reference to FIG. 6,FIG. 7, and FIGS. 8A and 8B. Note that the portions which are the sameas those in FIG. 2 and FIG. 3 are described using the same referencenumerals in the description for FIG. 6, FIG. 7, and FIGS. 8A and 8B.

FIG. 6 illustrates an example of a plan view of a pixel, correspondingto the circuit diagram of FIG. 3. In addition, FIG. 8A illustrates astate at the time before an electrode of a photodiode is formed. Notethat a cross-sectional view taken along a chain line A-B and across-sectional view taken along a chain line C-D in FIG. 6 correspondto FIG. 8A.

First, a conductive film is formed over a substrate 230. Then, gatesignal lines 207, 213, and 227, a capacitor wiring 224, a photodiodereset signal line 208, a reading signal line 209, and a photo sensorreference signal line 212 are formed through a first photolithographystep with the use of a first light-exposure mask. In this embodiment, aglass substrate is used as the substrate 230.

An insulating film serving as a base film may be provided between thesubstrate 230 and the conductive film. The base film has a function ofpreventing diffusion of an impurity element from the substrate 230. Thebase film can be formed with a single-layer structure or a layeredstructure including one or more of a silicon nitride film, a siliconoxide film, a silicon nitride oxide film, and a silicon oxynitride film.

The conductive film can be formed with a single-layer structure or alayered structure including a metal material such as molybdenum,titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium,or an alloy material which contains any of these metal materials as itsmain component.

Then, an insulating layer covering these wirings is formed, andselective etching is performed through a second photolithography stepwith the use of a second light-exposure mask so that an insulating layer231 remains only in a portion intersecting a wiring which is to beformed later. In this embodiment, a silicon oxynitride film with athickness of 600 nm is used as the insulating layer 231.

Next, a gate insulating layer 232 and an oxide semiconductor film areformed, and then, a first oxide semiconductor layer 233, a second oxidesemiconductor layer 253, a third oxide semiconductor layer 255, and afourth oxide semiconductor layer 256 are formed through a thirdphotolithography step with the use of a third light-exposure mask. Thefirst oxide semiconductor layer 233, the second oxide semiconductorlayer 253, the third oxide semiconductor layer 255, and the fourth oxidesemiconductor layer 256 overlap with the gate signal line 227, the gatesignal line 207, the reading signal line 209, and the gate signal line213, respectively with the gate insulating layer 232 providedtherebetween. In this embodiment, a silicon oxynitride film with athickness of 100 nm is used as the gate insulating layer 232, and anIn—Ga—Zn—O film with a thickness of 30 nm is used as the oxidesemiconductor film.

An oxide thin film represented by a chemical formula InMO₃(ZnO)_(m)(m>0) can be used for the first oxide semiconductor layer 233, thesecond oxide semiconductor layer 253, the third oxide semiconductorlayer 255, and the fourth oxide semiconductor layer 256. Here, Mrepresents one or more metal elements selected from Ga, Al, Mn, and Co.For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.Further, SiO₂ may be contained in the above oxide thin film.

As the target for forming the oxide thin film by a sputtering method,for example, an oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] is used to form an In—Ga—Zn—O film.Without limitation on the material and the component of the target, forexample, an oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] may be used. Note that in thisspecification, for example, an In—Ga—Zn—O film means an oxide filmincluding indium (In), gallium (Ga), and zinc (Zn), and there is noparticular limitation on the stoichiometric proportion.

Next, the oxide semiconductor layers are subjected to first heattreatment. The oxide semiconductor layers can be dehydrated ordehydrogenated by this first heat treatment. The temperature of thefirst heat treatment is higher than or equal to 400° C. and lower thanor equal to 750° C., preferably higher than or equal to 400° C. andlower than the strain point of the substrate. In embodiment, a rapidthermal anneal (RTA) apparatus is used, heat treatment is performed in anitrogen atmosphere at 650° C. for six minutes, the substrate isintroduced, without exposure to the air, into an electric furnace thatis one kind of a heat treatment apparatus, and heat treatment isperformed in a nitrogen atmosphere at 450° C. for one hour for the oxidesemiconductor layers. Then, the substrate is transferred into thedeposition chamber of the oxide semiconductor layers so as not to beexposed to the air in order to prevent mixing of water or hydrogen tothe oxide semiconductor layers, whereby the oxide semiconductor layersare obtained.

Next, the gate insulating layer 232 is selectively removed through afourth photolithography step with the use of a fourth light-exposuremask, so that an opening reaching the gate signal line 213 and anopening reaching the photodiode reset signal line 208 are formed.

Next, a conductive film is formed over the gate insulating layer 232 andthe oxide semiconductor layer. The conductive film can be formed using ametal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W as its component, an alloy film containing a nitride of any ofthese elements as its component, an alloy film containing a combinationof any of these elements, or the like. In this embodiment, theconductive film has a three-layer structure in which a Ti film with athickness of 100 nm, an Al film with a thickness of 400 nm, and a Tifilm with a thickness of 100 nm are stacked. Then, a resist mask isformed over the conductive film through a fifth photolithography stepusing a fifth light-exposure mask and etching is selectively performed,thereby forming a video data signal line 210, a photo sensor outputsignal line 211, and electrode layers 234, 235, 254, 257, 258, and 259.

Note that a transistor 221 illustrated in FIG. 3 includes the firstoxide semiconductor layer 233 and the electrode layer 234 serving as asource electrode layer or a drain electrode layer as illustrated in FIG.6. In addition, as illustrated in FIG. 6 and FIG. 8A, the electrodelayer 234, the gate insulating layer 232 serving as a dielectric, andthe capacitor wiring 224 form a storage capacitor 222. Further, asillustrated in FIG. 6, a transistor 201 includes the second oxidesemiconductor layer 253 and the electrode layer 254 serving as a sourceelectrode layer or a drain electrode layer.

Furthermore, a transistor 206 that is one of the elements included in aphoto sensor 106 in FIG. 3 includes the third oxide semiconductor layer255 and the electrode layer 257 serving as a source electrode layer or adrain electrode layer as illustrated in FIG. 6. In addition, atransistor 205 includes the fourth oxide semiconductor layer 256 and theelectrode layer 257 or the electrode layer 258 which serves as a sourceelectrode layer or a drain electrode layer as illustrated in FIG. 6. Asillustrated in FIG. 8A, the gate signal line 213 of the transistor 205is electrically connected to the electrode layer 236 through the openingof the gate insulating layer 232.

Next, second heat treatment is performed in an inert gas atmosphere oroxygen gas atmosphere (preferably at a temperature higher than or equalto 200° C. and lower than or equal to 400° C., for example, higher thanor equal to 250° C. and lower than or equal to 350° C.). In thisembodiment, the second heat treatment is performed at 300° C. for onehour under a nitrogen atmosphere. Through the second heat treatment,part of the oxide semiconductor layer (a channel formation region) isheated while being in contact with the insulating layer.

Next, an insulating layer 237 serving as a protective insulating layeris formed, and a sixth photolithography step is performed with the useof a sixth light-exposure mask, so that an opening reaching theelectrode layer 235, an opening reaching the electrode layer 234, and anopening reaching the electrode layer 236 are formed. In this embodiment,a silicon oxide film with a thickness of 300 nm obtained by a sputteringmethod is used as the insulating layer 237.

Next, a p-layer 238, an i-layer 239, and an n-layer 240 are stacked by aplasma CVD method. In this embodiment, a microcrystalline silicon filmcontaining boron with a thickness of 60 nm is used as the p-layer 238,an amorphous silicon film with a thickness of 400 nm is used as thei-layer 239, and a microcrystalline silicon film containing phosphoruswith a thickness of 80 nm is used as the n-layer 240. The p-layer 238,the i-layer 239, and the n-layer 240 are selectively etched through aseventh photolithography step with the use of a seventh light-exposuremask, and further, as illustrated in FIG. 8A, the periphery of then-layer 240 and part of the i-layer 239 are selectively removed. FIG. 8Ais a cross-sectional view up to this stage and a plan view thereofcorresponds to FIG. 6.

Next, an eight photolithography step is performed in which aphotosensitive organic resin layer is formed, a region in which anopening is to be formed is exposed to light with the use of an eighthlight-exposure mask, a region to have an uneven shape is exposed tolight with the use of a ninth light-exposure mask, development isperformed, and an insulating layer 241 partly having an uneven shape isformed. In this embodiment, an acrylic resin with a thickness of 1.5 μmis used as the photosensitive organic resin layer.

Then, a conductive film having reflectivity is deposited, and a ninthphotolithography step is performed with the use of a tenthlight-exposure mask, so that a reflective electrode layer 242 and aconnection electrode layer 243 are formed. Note that the reflectiveelectrode layer 242 and the connection electrode layer 243 areillustrated in FIG. 8B. Al, Ag, or an alloy thereof such as aluminumcontaining Nd or an Ag—Pd—Cu alloy is used as the conductive film havingreflectivity. In this embodiment, a stack of a Ti film with a thicknessof 100 nm and an Al film with a thickness of 300 nm provided thereoveris used as the conductive film having reflectivity. After the ninthphotolithography step, third heat treatment is performed. In thisembodiment, the third heat treatment is performed at 250° C. for onehour in a nitrogen atmosphere.

Through the above steps, a transistor electrically connected to thereflective electrode layer 242 and a photodiode electrically connectedto the gate signal line 213 through the connection electrode layer 243can be formed over one substrate through the nine photolithography stepswith the use of the ten light-exposure masks in total.

Note that a cross-sectional photograph of the periphery portion of thephotodiode is shown in FIG. 11A. FIG. 11A shows a cross section of aphotodiode 204 and FIG. 11B illustrates a schematic view of the crosssection.

An alignment film 244 covering the reflective electrode layer 242 isformed. A cross-sectional view at this stage corresponds to FIG. 8B.Note that the same reference numeral is used for common parts in FIG. 8Band FIG. 11B. Thus, an active matrix substrate can be manufactured.

A counter substrate to be bonded to the active matrix substrate isprepared. Over the counter substrate, a light-blocking layer (alsoreferred to as a black matrix) and a light-transmitting conductive filmare formed and a columnar spacer using an organic resin is formed. Then,an alignment film is formed to cover them.

The counter substrate is attached to the active matrix substrate with asealant, and a liquid crystal layer is sandwiched between the pair ofsubstrates. The light-blocking layer of the counter substrate isprovided so as not to overlap with a display region of the reflectiveelectrode layer 242 and a sensing region of the photodiode. The columnarspacer provided on the counter substrate is positioned so as to overlapwith the electrode layers 251 and 252. Since the columnar spaceroverlaps with the electrode layers 251 and 252, the pair of substratesis held at a certain distance. The electrode layers 251 and 252 can beformed in the same step as that of the electrode layer 234; thus, it isnot necessary to increase the number of masks. The electrode layers 251and 252 are not electrically connected to anywhere and are in a floatingstate.

A plan view of the pixels for the pair of substrate which are attachedto each other in this manner corresponds to FIG. 7. In FIG. 7, a regionwhich does not overlap with the black matrix serves as a light-receivingregion of a photo sensor and a reflective electrode region. Theproportion of the area of the reflective electrode in a unit area (120μm×240 μm) illustrated in FIG. 7 is 77.8%. The area of thelight-receiving region of the photo sensor is approximately 1140 μm². Inaddition, since the reflective electrode layer 242 is provided over thephotosensitive organic resin layer having an uneven portion, thereflective electrode layer 242 has a random plane pattern as illustratedin FIG. 7. The surface shape of the photosensitive organic resin layeris reflected on a surface of the reflective electrode layer 242 so thatthe surface of the reflective electrode layer 242 has an uneven shape;thus, specular reflection is prevented. Note that in FIG. 7, a recessedportion 245 of the reflective electrode layer 242 is also illustrated.The periphery of the recessed portion 245 is positioned inside theperiphery of the reflective electrode layer, and the photosensitiveorganic resin layer below the recessed portion 245 has a smallerthickness than other regions.

If necessary, a surface of the counter substrate where external lightenters may be provided with an optical film such as a retardation filmfor adjusting phase difference, a film having a polarization function,an anti-reflection plate, or a color filter.

FIG. 12 shows a photograph of a panel which was actually manufactured,in which a video signal is input to the panel through an FPC to performdisplay. A half screen of the panel displayed a still image and theother half of the screen displayed a moving image. When the half of thescreen of the panel displays a still image and the other half of thescreen displays a moving image, power consumption can be reduced. Inaddition, keyboard buttons are displayed as illustrated in FIG. 1A bytouching the screen with a finger and data is input by touching portionswhere keyboard buttons are displayed with fingers, whereby letterscorresponding to the keyboard buttons can be displayed on the displayregion. Further, when photo sensors are used and a shadow of a fingercan be sensed with a sufficient amount of external light even in anon-contact state in which the finger is close to the screen but nottouches the screen, screen operation without contact can be performed.

Embodiment 3

In this embodiment, an example of a liquid crystal display modulecapable of full-color display in which a color filter is provided willbe described.

FIG. 9 illustrates a structure of a liquid crystal display module 190.The liquid crystal display module 190 includes a display panel 120 inwhich liquid crystal elements are provided in a matrix, and a polarizingplate and a color filter 115 which overlap with the display panel 120.In addition, flexible printed circuits (FPCs) 116 a and 116 b serving asexternal input terminals are electrically connected to a terminalportion provided in the display panel 120. The display panel 120 has thesame structure as the display panel 100 described in Embodiment 1. Notethat since full-color display is employed, the display panel 120 usesthree display elements of a red display element, a green displayelement, and a blue display element and has a circuit configuration inwhich the three display elements are supplied with respective videosignals different from each other.

Further, FIG. 9 schematically illustrates a state in which externallight 139 is transmitted through the liquid crystal elements over thedisplay panel 120 and reflected at the reflective electrode. Forexample, in a pixel overlapping with a red region of the color filter,the external light 139 is transmitted through the color filter 115 andthen through the liquid crystal layer, reflected at the reflectiveelectrode, and transmitted again through the color filter 115 to beextracted as red light. FIG. 9 schematically illustrates light 135 ofthree colors by arrows (R, G, and B). The intensity of the light whichis transmitted through the liquid crystal elements is modulated by animage signal. Therefore, a viewer can capture an image by reflectionlight of the external light 139.

In addition, the display panel 120 includes photo sensors and has atouch-input function. When the color filter also overlaps with alight-receiving region of photo sensors, the display panel 120 can havea function of a visible light sensor. Further, in order to improve theoptical sensitivity of the photo sensor, a large amount of incidentlight is taken in. Therefore, an opening may be provided in the colorfilter in a region overlapping with the light-receiving region of photosensors so that the light-receiving region of photo sensors and thecolor filter do not overlap with each other.

This embodiment can be freely combined with Embodiment 1 or Embodiment2.

Embodiment 4

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of the above embodimentsis described.

FIG. 10A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader is provided with thesolar cell 9633 and a display panel so that the solar cell 9633 and thedisplay panel can be opened and closed freely. In the electronic bookreader, power from the solar cell is supplied to the display panel or avideo signal processing portion. The electronic book reader illustratedin FIG. 10A can have a function of displaying various kinds of data(e.g., a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the informationdisplayed on the display portion by touch input, a function ofcontrolling processing by various kinds of software (programs), and thelike. Note that in FIG. 10A, a structure including a battery 9635 and aDCDC converter (hereinafter abbreviated as a converter 9636) isillustrated as an example of the charge and discharge control circuit9634.

The display portion 9631 is a reflective liquid crystal display devicehaving a touch-input function with the use of photo sensors and is usedin a comparatively bright environment. Therefore, the structureillustrated in FIG. 10A is preferable because power generation by thesolar cell 9633 and charge in the battery 9635 can be performedeffectively. Note that a structure in which the solar cell 9633 isprovided on each of a surface and a rear surface of the housing 9630 ispreferable in order to charge the battery 9635 efficiently. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 10A are described with reference to ablock diagram in FIG. 10B. The solar cell 9633, the battery 9635, theconverter 9636, the converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 10B, and the battery 9635, the converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the converter9636 so that the power has a voltage for charging the battery 9635.Then, when the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 so as to be avoltage needed for the display portion 9631. In addition, when displayon the display portion 9631 is not performed, the switch SW1 is turnedoff and a switch SW2 is turned on so that charge of the battery 9635 maybe performed.

Note that although the solar cell 9633 is described as an example of ameans for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar cell 9633 andanother means for charge may be used.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 5

In this embodiment, an example in which a transistor and a photo sensorare formed over a glass substrate, and transferred to and mounted on aflexible substrate. Note that here, cross-sectional views of steps forforming the transistor are illustrated in FIGS. 13A to 13C. In FIGS. 13Ato 13C, the same steps as those of Embodiment 2 and detail descriptionof the structure of a photodiode or the like are omitted and the sameportions as those of FIGS. 8A and 8B are denoted by the same referencenumerals.

First, a separation layer 260 is deposited over a substrate 230 by asputtering method, and an oxide insulating film 261 functioning as abase film is formed thereover. Note that as the substrate 230, a glasssubstrate, a quartz substrate, or the like is used. The oxide insulatingfilm 261 is formed using a material such as silicon oxide, siliconoxynitride (SiO_(x)N_(y)) (x>y>0), or silicon nitride oxide(SiN_(x)O_(y))(x>y>0) by a PCVD method, a sputtering method, or thelike.

The separation layer 260 may be formed using a metal film, a layeredstructure of a metal film and a metal oxide film, or the like. As ametal film, either a single layer or stacked layers of a film formedusing an element selected from tungsten (W), molybdenum (Mo), titanium(Ti), tantalum (Ta), niobium (Nb), nickel (Ni), cobalt (Co), zirconium(Zr), zinc (Zn), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium(Os), and iridium (Ir), or a film formed using an alloy material or acompound material containing the element as its main component. Forexample, when a tungsten film is provided as a metal film by asputtering method, a CVD method, or the like, a metal oxide film formedof tungsten oxide can be formed on a surface of the tungsten film byapplication of plasma treatment to the tungsten film. In addition, forexample, after a metal film (e.g., tungsten) is formed, an insulatingfilm formed of silicon oxide or the like may be formed over the metalfilm by a sputtering method, and also metal oxide (e.g., tungsten oxideover tungsten) may be formed over the metal film. Further, high-densityplasma treatment with the use of a high-density plasma apparatus may beperformed as the plasma treatment. Furthermore, besides a metal oxidefilm, a metal nitride film or a metal oxynitride film may be used. Inthis case, plasma treatment or heat treatment is performed on the metalfilm in a nitrogen atmosphere or an atmosphere of nitrogen and oxygen.

Next, a conductive film is formed over the oxide insulating film 261.Then, in a similar manner to Embodiment 2, a gate signal line 227, acapacitor wiring 224, a photodiode reset signal line, a reading signalline, and a photo sensor reference signal line are formed through afirst photolithography step with the use of a first light-exposure mask.

Subsequent steps are performed in accordance with Embodiment 2, wherebythe transistor and a reflective electrode layer 242 are formed. Then,the reflective electrode layer 242 is covered with a water-soluble resinlayer 262. FIG. 13A is a cross-sectional view at this stage. Note that,for simplification, a cross-sectional structure of the periphery of thereflective electrode layer 242 is illustrated but a photodiode formedover the same substrate is not illustrated in FIG. 13A.

Next, the water-soluble resin layer 262 is fixed to a supportingsubstrate or the like, and an opening is formed by irradiation of theseparation layer with laser light or the like, whereby a layer includingthe transistor is separated from the substrate 230. FIG. 13B is across-sectional view at this stage. As illustrated in FIG. 13B,separation is performed at an interface between the separation layer 260provided with the substrate 230 and the oxide insulating film 261.

Then, as illustrated in FIG. 13C, a flexible substrate 264 is attachedto a surface of the layer including the transistor (a surface which isexposed by the separation) with an adhesive layer 263. As the flexiblesubstrate 264, a plastic film or a thin stainless-steel substrate can beused.

Next, the water-soluble resin layer 262 is removed and an alignment film244 is formed. Then, a counter substrate 268 including a counterelectrode 267 and the flexible substrate 264 are attached to each otherwith a sealant. Note that before the attachment, an alignment film 266covering the counter electrode 267 is formed for the counter substrate268. In the case of using a liquid crystal dropping method, liquidcrystal is chopped onto a region surrounded by a sealant of a closedloop and attachment of the pair of substrates is performed under reducedpressure. In this manner, a region surrounded by the pair of substratesand the sealant is filled with a liquid crystal layer 265. When aplastic film with high light-transmitting properties and smallretardation is used as the counter substrate 268, a flexible liquidcrystal panel can be manufactured.

The above-described process for manufacturing a flexible liquid crystalpanel is only one example. Alternatively, for example, a flexible liquidcrystal panel may be manufactured in such a manner that glass substrateswhich are used as the substrate 230 and the counter substrate 268 arethinned by polishing or the like after the transistor is manufactured.In the case of thinning by polishing, both of the substrate 230 and thecounter substrate 268 are thinned by polishing after filling with theliquid crystal layer is performed.

FIGS. 14A and 14B illustrate an example of an electronic book readerusing a liquid crystal panel.

FIGS. 14A and 14B as an example of an electronic book reader illustratethe case where a supporting portion 4308 is provided at an end portionof a liquid crystal panel 4311. The specific structure of the electronicbook reader is described below with reference to FIGS. 14A and 14B. FIG.14A illustrates the electronic book reader which is horizontallydisposed, and FIG. 14B illustrates the electronic book reader which isvertically disposed.

The electronic book reader illustrated in FIGS. 14A and 14B includes theflexible liquid crystal panel 4311 including a display portion 4301, thesupporting portion 4308 provided at the end portion of the liquidcrystal panel 4311, a scan line driver circuit 4321 a for controllingthe display of the display portion 4301, a photo sensor driver circuit4321 b for controlling a photodiode provided in the display portion4301, and a signal line driver circuit 4323 for controlling the displayof the display portion 4301.

The scan line driver circuit 4321 a and the photo sensor driver circuit4321 b are provided over a flexible surface of the liquid crystal panel4311, and the signal line driver circuit 4323 is provided inside thesupporting portion 4308.

It is preferable that the supporting portion 4308 be less flexible (morerigid) than at least the liquid crystal panel 4311. For example, ahousing forming the supporting portion 4308 can be formed using plastic,metal, or the like which is thicker than the liquid crystal panel 4311.In that case, the electronic book reader can be bent (warped) in aportion other than the supporting portion 4308.

There is no particular limitation on where to provide the supportingportion 4308. For example, the supporting portion 4308 can be providedalong the end portion of the liquid crystal panel 4311. For example, asshown in FIGS. 14A and 14B, in the case where the liquid crystal panel4311 has a rectangular shape, the supporting portion 4308 can beprovided along a predetermined side of the liquid crystal panel 4311 (sothat the side is fixed). Note that the “rectangular shape” here includesa shape in which a corner of the rectangular is rounded.

Further, when the scan line driver circuit 4321 a, the photo sensordriver circuit 4321 b, and a pixel circuit included in the displayportion 4301 are formed over the substrate having flexibility throughthe same process, the scan line driver circuit 4321 a and the photosensor driver circuit 4321 b can be bent and a reduction in cost can beachieved.

The pixel circuit included in the display portion 4301 and elementsincluded in the scan line driver circuit 4321 a and the photo sensordriver circuit 4321 b can be formed using thin film transistors or thelike. On the other hand, a high-speed operation circuit such as thesignal line driver circuit 4323 can be formed using all integratedcircuit (IC) formed using a semiconductor substrate such as a siliconsubstrate or an SOI substrate, and the IC can be provided inside thesupporting portion 4308.

The liquid crystal panel 4311 is formed using a flexible substrate, andtherefore, data can be input without any problem even when the screen isbent because of touch input with the use of a photodiode. Thus, theoperability of the liquid crystal panel 4311 is better than otherelectronic devices having a touch input system.

Note that an example in which a metal layer is used as the separationlayer in this embodiment; however, an embodiment is not limited thereto.A separation method with the use of ablation with laser irradiation, aseparation method with the use of an organic resin, or the like can beused.

Embodiment 6

In this embodiment, an example of the structure of a scan line drivercircuit of a liquid crystal panel is described with reference todrawings.

FIG. 15 is a block diagram of a driver circuit described in thisembodiment. In the case where the driver circuit is used as a gatedriver (scan line driver circuit) of VGA, it is necessary to drive 480gate lines, and therefore, data lines for 9 bits are needed. In thisexample, an example of 3 bits is described.

A block 701 denotes a circuit that generates a gate signal of a firststage, a block 702 denotes a circuit that generates a gate signal of asecond stage, a block 703 denotes a circuit that generates a gate signalof a third stage, a block 704 denotes a circuit that generates a gatesignal of a fourth stage, and a block 705 denotes a circuit thatgenerates a gate signal of a fifth stage. In the case of VGA, there arealso blocks that generate gate signals of 6th to 480th stages inaddition to the above.

Data0 a, Data0 b, Data1 a, Data1 b, Data2 a, and Data2 b denote datalines. Character of any of 0 to 2 which is the second from the lastcharacter of a signal name corresponds to 3-bit data. As for the lastcharacters “a” and “b” of the signal names, “b” corresponds to aninverted signal of “a” but is not a completely-inverted signal. Data0 ato Data2 b are set to 0 (low, also referred to as GND) in a period inwhich data is not input. A method for connecting data lines and blocksin respective stages is as follows: when the first stage represents“001” in binary, since Data0 a or Data0 b corresponds to a lowest bitand a lowest bit of the first stage is “1”; therefore, the block of thefirst stage is connected to one whose last character is “b”, i.e., Data0b. In a similar manner, since a bit next to the lowest bit is “0”, thefirst stage is connected to one whose last character is “a”, i.e., theData1 a.

FIG. 16 shows a relation among data lines with respect to time. In aperiod 1 in which the block 701 of the first stage is selected, Data2 ais set to 0 (low), Data1 a is set to 0 (low), and Data0 a is set to 1(high), which corresponds to “001” that is binary representation of 1.In a period 2 in which the block 702 of the second stage is selected,Data2 a is set to 0 (low), Data1 a is set to 1 (high), and Data0 a isset to 0 (low), which corresponds to “010” that is binary representationof 2. As for the third stages and subsequent stages, data are alsodetermined in the same manner.

In addition, when the Data0 a is 0 (low), Data0 b corresponding theretois set to 1 (high) inversely, whereas when Data0 a is 1 (high), Data0 bis set to 0 (low). The same relation between Data1 a and Data2 a appliesto Data1 b and Data2 b. Note that a period in which both of Data0 a andData0 b are 0 (low) is inserted between the period 1 in which the block701 of the first stage is selected and the period 2 in which the block702 of the second stage is selected and serves as a period in which datais not input.

FIG. 17 is a diagram of a circuit forming the inside of the block 701.The same applies to circuits forming the insides of the block 702, theblock 703, the block 704, and the block 705. Also in FIG. 17, an exampleof 3 bits is described. An n-channel transistor group 802, an n-channeltransistor 803, an n-channel transistor 804, and an n-channel transistorgroup 806 which are illustrated in FIG. 17 are formed over the samesubstrate as that of a transistor of a display portion, and an oxidesemiconductor layer is used as a channel in each of the transistorgroups and the transistors.

Data0 in FIG. 15 and Data0 in FIG. 17 correspond to each other, andData0 is electrically connected to Data0 a or Data0 b of FIG. 15. A node801 has a function of holding data. Although data may be held by acapacitor, since parasitic capacitance is acceptable, a capacitor forholding data is omitted in this embodiment. When one of data lines Data0to Data2 is set to 1 (high), one of transistors of the n-channeltransistor group 802 is turned on and the node 801 is set to 0 (low).When all of the data lines Data0 to Data2 are 0 (low), the node 801 iskept at 1 (high) and this block is regarded as being selected. Further,in a period in which data is not input, all of the data lines Data0 toData2 are set to 0 (low) and the whole transistor group 802 is turnedoff.

In order set the node 801 to 1 (high), a reset signal is set to 1(high), whereby the transistor 803 that is an n-channel transistor isturned on. Note that even when the transistor 803 is turned on, the node801 does not always have the same potential as VDD because of thethreshold value of the transistor 803, which does not cause a problem.When a period in which the reset signal is 1 (high) does not overlapwith a period in which one of the data lines Data0 to Data2 is 1 (high),increase of a current flowing from a power supply VDD to GND can beprevented.

When all the data lines are 0 (low) in a period of data input, that is,when the block is selected, the node 801 is kept at 1 (high) and thetransistor 804 that is an n-channel transistor is turned on. Further,when this block is selected, the node 801 is not electrically connectedto the power supply VDD and GND. When a write signal is changed from 0(low) to 1 (high), the potential of the node 801 is raised due tocapacitive coupling of a capacitor 805. A circuit of the capacitor 805is referred to as a bootstrap circuit.

After being raised, the potential of the node 801 is preferably higherthan a potential obtained by adding the threshold value of thetransistor 804 to the highest potential of the write signal. After thepotential of the node 801 is raised, if the potential of the node 801 ishigher than the potential obtained by adding the threshold value of thetransistor 804 to the highest potential of the write signal, thecapacitor 805 is not needed or parasitic capacitance is enough in somecases.

After the potential of the node 801 is raised, in the case where thepotential of the node 801 is lower than the potential obtained by addingthe threshold value of the transistor 804 to the highest potential ofthe write signal, there is a possibility that the potential of a nodeOut is not increased to the highest potential of the write signal andwriting to a pixel is not performed in time. When the potential of thenode 801 is higher than the potential obtained by adding the thresholdvalue of the transistor 804 to the highest potential of the writesignal, the write signal is change from 0 (low) to 1 (high), and thenode Out is also changed from 0 (low) to 1 (high). The node Out isconnected to a gate line of a pixel. Next, when the write signal ischanged from 1 (high) to 0 (low), the potential of the node 801 islowered due to capacitive coupling of the capacitor 805; however, thepotential of the node 801 is approximately equal to the potential of VDDsupplied by the reset signal and the transistor 804 is not tuned off. Inother words, since the potential of the node 801 is higher than apotential obtained by adding the threshold value of the transistor 804to the level of 0 (low) of the write signal, the node Out is also 0(low).

When one of the data lines is 1 (high) in a period of data input, thatis, when the block is not selected, the node 801 is 0 (low) and thetransistor 804 is turned off.

Even when the write signal is changed from 0 (low) to 1 (high), sincethe transistor group 802 is turned on, the potential of the node 801 is0 (low) and the transistor 804 is turned off. Since when the transistorgroup 802 has a low ability to pass current, the potential of the node801 is raised due to capacitive coupling of the capacitor 805;therefore, it is necessary that the ability to pass current of thetransistor group 802 is determined so that change of the node Out issmall.

Next, even when the write signal is changed from the 1 (high) to 0(low), since the transistor group 802 is turned on, the potential of thenode 801 is 0 (low) and the transistor 804 is turned off. When thetransistor group 802 has a low ability to pass current, the potential ofthe node 801 is lowered due to capacitive coupling of the capacitor 805;therefore, it is necessary that the ability to pass current of thetransistor group 802 is determined so that change of the node Out issmall.

In the case where the transistor group 806 is not provided, when one ofthe data lines is 1 (high) in a period of data input, that is, when thisblock is not selected, the node Out is not electrically connected to thepower supply and may be influenced by a video signal. The potential ofthe node Out is also changed by capacitive coupling between a sourceelectrode and a drain electrode of the transistor 804. Therefore, in aperiod in which the transistor 804 is turned off, the node Out ispreferably fixed at 0 (low).

FIG. 18 is diagram of time and potentials of nodes. Operation as for thecircuit diagram of FIG. 17 is described using FIG. 18.

In a period 901, the node 801 is set to 1 (high) by the reset signal. Inthe period 901 although the transistor 804 is turned on, the writesignal is 0 (low) and therefore, the potential of the node Out is also 0(low). A next period 902 is a period until the beginning of data input,in which the reset signal is set to 0 (low). It is preferable thatincrease of current flowing from the power supply VDD to GND beprevented by providing the period 902.

A next period 903 is needed for determining the potential of the node801 by input data, and the case where all of the data lines Data0 toData2 are set to 0 (low) is described here. In the beginning of a nextperiod 904, the write signal is set to 1 (high) and the potential of thenode 801 is raised. In the period 904, the potential of the node Out isalso set to 1 (high).

In a next period 905, the write signal is set to 0 (low). Although thepotential of the node 801 is also lowered, since the transistor 804 isturned on, the potential of the node Out is 0 (low). In a next period906, the period of data input is terminated, and in a next period 907,the reset signal is set to 1 (high) again and the potential of the node801 is set to 1 (high). The above is one horizontal period, and afterthat, the horizontal period is repeated. A period which is provided inplace of the period 903 and in which one of the data lines Data0 toData2 is set to 1 (high) is denoted by a period 908. In the period 908,when one of the data lines Data0 to Data2 is set to 1 (high), thepotential of the node 801 is set to 0 (low). If the period 908 is short,and the write signal is set to 1 (high) in a next period 909 before thepotential of the node 801 is sufficiently lowered by turning off thetransistor 804, there is possibility that the potential of the node Outcannot be fixed only by the transistor group 806 and is increasedtemporarily. If the transistor 804 is turned off in the period 908, thenode Out is kept at 0 (low) even when the write signal is set to (high)in the next period 909. Note that the potential of the node Out tends toincrease due to parasitic capacitance between the source electrode andthe drain electrode of the transistor 804 even when the transistor 804is turned off. It is necessary that the potential of the node Out isadjusted by the transistor group 806 in order not to turning on thepixel transistor. A load of a gate line is large, and therefore, theparasitic capacitance between the source electrode and the drainelectrode does not cause a large problem.

In this embodiment. “1 (high)” is described as the potential of VDD and“0 (low)” is described as the potential of GND. When the highestpotential of the write signal is lower than VDD, a bootstrap circuit isnot needed, but it is not preferable that two kinds of power suppliesare provided on the outside because cost is increased. It is possible toperform operation using one power supply in this embodiment.

Note that the structure of the driver circuit of the display devicedescribed in this embodiment can be implemented in free combination withany of structures in other embodiments in this specification.

In the structure of the driver circuit of the display device describedin this embodiment, when the write signal and each of the data lines areset to 0 (low) and the reset signal is set to 1 (high), gate lines ofall of the pixels are set at 0 (low). Unlike a case where a shiftregister circuit is used for a driver circuit of a display portion, agate line of a pixel in an arbitrary row can be set to 1 (high) in anarbitrary order.

This embodiment is one example of a decoder circuit included in a drivercircuit, and therefore, an embodiment is not limited to the structure ofthe driver circuit of the display device described in this embodiment.

This application is based on Japanese Patent Application serial No.2010-050947 filed in Japan Patent Office on Mar. 8, 2010, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

-   100: display panel; 101: pixel circuit; 103: pixel; 104: pixel; 105:    display element; 106: photo sensor: 107: display element driver    circuit on a signal line side; 108: display element driver circuit    on a scan line driver circuit; 109: photo sensor reading circuit;    110: photo sensor driver circuit; 115: color filter; 116 a: FPC; 116    b: FPC; 120: display panel; 125: display element; 135: light; 139:    external light; 190: liquid crystal display module; 201: transistor;    202: storage capacitor; 203: liquid crystal element; 204:    photodiode; 205: transistor; 206: transistor; 207: gate signal line;    208: photodiode reset signal line; 209: signal line; 210: video data    signal line; 211: photo sensor output signal line; 212: photo sensor    reference signal line; 213: gate signal line; 214: capacitor wiring;    221: transistor; 222: storage capacitor; 223: liquid crystal    element; 224: capacitor wiring; 227: gate signal line; 230:    substrate; 231: insulating layer; 232: gate insulating layer; 233:    oxide semiconductor layer; 234: electrode layer; 235: electrode    layer; 236: electrode layer; 237: insulating layer; 238: p-layer;    239: i-layer; 240: n-layer; 241: insulating layer; 242: reflective    electrode layer; 243: connection electrode layer; 244: alignment    film; 245: recessed portion; 251: electrode layer; 253: oxide    semiconductor layer; 254: electrode layer; 255: oxide semiconductor    layer; 256: oxide semiconductor layer; 257: electrode layer; 258:    electrode layer; 260: separation layer; 261: oxide insulating film;    262: resin layer; 263: adhesive layer; 264: substrate; 265: liquid    crystal layer; 266: alignment film; 267: counter electrode; 268:    counter substrate; 701: block; 702: block; 703: block; 704: block;    705: block; 801: node; 802: transistor group; 803: transistor: 804:    transistor; 805: capacitor; 806: transistor group; 901: period; 902:    period; 903: period; 904: period; 905: period; 906: period; 907:    period; 908: period; 909: period; 1030: electronic device; 1031:    button; 1032: display portion; 1033: region; 1034: switch; 1035:    power supply switch; 1036: keyboard display switch; 4301: display    portion; 4308: supporting portion; 4308: supporting portion; 4311:    liquid crystal panel; 4321 a: scan line driver circuit; 4321 b:    photo sensor driver circuit; 4323: signal line driver circuit; 9630:    housing; 9631: display portion; 9632: operation key; 9633: solar    cell; 9634: charge and discharge control circuit; 9635: battery;    9636: converter; 9637: converter.

The invention claimed is:
 1. An electronic device comprising: a displayportion having a touch-input function, the display portion comprising,over a flexible substrate: a photo sensor; a reflective electrode layer;a first display element comprising a first transistor and a firstportion of the reflective electrode layer electrically connected to oneof a source electrode and a drain electrode of the first transistor; asecond display element comprising a second transistor and a secondportion of the reflective electrode layer electrically connected to oneof a source electrode and a drain electrode of the second transistor; afirst line electrically connected to a gate of the first transistor; asecond line electrically connected to a gate of the second transistor;and a third line electrically connected to the other of the sourceelectrode and the drain electrode of the first transistor and to theother of the source electrode and the drain electrode of the secondtransistor, wherein the photo sensor comprises: a photodiode; a thirdtransistor whose gate is electrically connected to the photodiode; and afourth transistor, wherein the first portion of the reflective electrodelayer and the second portion of the reflective electrode layer do notoverlap with each other, wherein the first portion of the reflectiveelectrode layer is not in direct contact with the second portion of thereflective electrode layer, wherein one of a source electrode and adrain electrode of the third transistor is electrically connected to afourth line, the other of the source electrode and the drain electrodeof the third transistor is electrically connected to one of a sourceelectrode and a drain electrode of the fourth transistor, and the otherof the source electrode and the drain electrode of the fourth transistoris electrically connected to a fifth line, wherein an oxidesemiconductor layer of the fourth transistor overlaps with the firstportion of the reflective electrode layer, and wherein the fourth lineoverlaps with the second portion of the reflective electrode layer. 2.The electronic device according to claim 1, wherein one of the sourceelectrode and the drain electrode of the third transistor overlaps withthe first portion of the reflective electrode layer and the other of thesource electrode and the drain electrode of the fourth transistoroverlaps with the second portion of the reflective electrode layer. 3.The electronic device according to claim 1, wherein an off current permicrometer of channel width of the fourth transistor is less than orequal to 10 aA.
 4. The electronic device according to claim 1, whereinthe oxide semiconductor layer comprises indium, gallium, zinc andoxygen.
 5. The electronic device according to claim 1, wherein the photosensor is positioned between the first portion of the reflectiveelectrode layer and the second portion of the reflective electrode layerwhen seen from above.
 6. The electronic device according to claim 1,further comprising a color filter overlapping with the first portion ofthe reflective electrode layer and the second portion of the reflectiveelectrode layer.
 7. The electronic device according to claim 1, whereinthe first portion of the reflective electrode layer and the secondportion of the reflective electrode layer are made of one selected fromthe group consisting of Al, Ag, and an alloy including one or both of Aland Ag.
 8. An electronic device comprising: a display portion having atouch-input function, the display portion comprising: a plurality ofdisplay elements each including a first transistor electricallyconnected to a reflective electrode layer, and a plurality of photosensors over a flexible substrate, wherein each of the plurality ofphoto sensors comprises: a photodiode; a second transistor whose gate iselectrically connected to the photodiode; and a third transistor whosegate is electrically connected to a first line, wherein the reflectiveelectrode layer of one of the plurality of display elements does notoverlap with that of another of the plurality of display elements,wherein the plurality of display elements is arranged in a matrix,wherein one of a source electrode and a drain electrode of the secondtransistor is electrically connected to a second line, the other of thesource electrode and the drain electrode of the second transistor iselectrically connected to one of a source electrode and a drainelectrode of the third transistor, and the other of the source electrodeand the drain electrode of the third transistor is electricallyconnected to a third line, wherein an oxide semiconductor layer of thethird transistor overlaps with the first line with a gate insulatinglayer provided therebetween, and wherein the first line overlaps withthe reflective electrode layer.
 9. The electronic device according toclaim 8, wherein an off current per micrometer of channel width of thethird transistor is less than or equal to 10 aA.
 10. The electronicdevice according to claim 8, wherein the oxide semiconductor layercomprises indium, gallium, zinc and oxygen.
 11. The electronic deviceaccording to claim 8, further comprising a color filter overlapping withthe reflective electrode layer.
 12. The electronic device according toclaim 8, wherein the reflective electrode layer is made of one selectedfrom the group consisting of Al, Ag, and an alloy including one or bothof Al and Ag.
 13. An electronic device comprising: a display portionhaving a touch-input function, the display portion comprising, over aflexible substrate: a photo sensor; a reflective electrode layer; afirst display element comprising a first transistor and a first portionof the reflective electrode layer electrically connected to one of asource electrode and a drain electrode of the first transistor; a seconddisplay element comprising a second transistor and a second portion ofthe reflective electrode layer electrically connected to one of a sourceelectrode and a drain electrode of the second transistor; a first lineelectrically connected to a gate of the first transistor, a second lineelectrically connected to a gate of the second transistor; and a thirdline electrically connected to the other of the source electrode and thedrain electrode of the first transistor and to the other of the sourceelectrode and the drain electrode of the second transistor, wherein thephoto sensor comprises: a photodiode; a third transistor whose gate iselectrically connected to the photodiode; and a fourth transistor whosegate is electrically connected to a sixth line, wherein the firstportion of the reflective electrode layer and the second portion of thereflective electrode layer do not overlap with each other, wherein thefirst portion of the reflective electrode layer is not in direct contactwith the second portion of the reflective electrode layer, wherein oneof a source electrode and a drain electrode of the third transistor iselectrically connected to a fourth line, the other of the sourceelectrode and the drain electrode of the third transistor iselectrically connected to one of a source electrode and a drainelectrode of the fourth transistor, and the other of the sourceelectrode and the drain electrode of the fourth transistor iselectrically connected to a fifth line, wherein an oxide semiconductorlayer of the fourth transistor overlaps with the first portion of thereflective electrode layer, wherein the fourth line overlaps with thesecond portion of the reflective electrode layer, wherein the oxidesemiconductor layer of the fourth transistor overlaps with the sixthline with a gate insulating layer provided therebetween, and wherein thesixth line overlaps with the first portion of the reflective electrodelayer.
 14. The electronic device according to claim 13, wherein an offcurrent per micrometer of channel width of the fourth transistor is lessthan or equal to 10 aA.
 15. The electronic device according to claim 13,wherein the oxide semiconductor layer of each of the third transistorand the fourth transistor comprises indium, gallium, zinc and oxygen.16. The electronic device according to claim 13, wherein the photosensor is positioned between the first portion of the reflectiveelectrode layer and the second portion of the reflective electrode layerwhen seen from above.
 17. The electronic device according to claim 13,further comprising a color filter overlapping with the first portion ofthe reflective electrode layer and the second portion of the reflectiveelectrode layer.
 18. The electronic device according to claim 13,wherein one of the source electrode and the drain electrode of the thirdtransistor overlaps with the first portion of the reflective electrodelayer and the other of the source electrode and the drain electrode ofthe fourth transistor overlaps with the second portion of the reflectiveelectrode layer.
 19. The electronic device according to claim 13,wherein the first portion of the reflective electrode layer and thesecond portion of the reflective electrode layer are made of oneselected from the group consisting of Al, Ag, and an alloy including oneor both of Al and Ag.